Depends how you want it...use your hand to compare the way you want it...
Explanation:
Touch
Thumb-Pinky Finger = Well done
Thumb-Middle Finger = Medium
Thumb-Index Finger = Rare
You touch the area of your hand below your thumb, for each finger you use, you will feel a different firmness...play around, you will see what I am talking about, its harder to explain in words but it's really simple.
Touch your hand, touch the steak, once they feel about the same, your steak is ready
This is a master way of cooking the steak, passed down from the master himself which will remain annonymous...